code: def test_sum_string():
      s = Series(["Hello", "there", "World"])
  
      got = s.sum()
      expected = "HellothereWorld"
  
      assert got == expected
  
      s = Series(["Hello", None, "World"])
  
      got = s.sum()
      expected = "HelloWorld"
  
      assert got == expected
  
imports: from decimal import Decimal
  from itertools import product
  import numpy
  import pandas
  import pytest
  import cudf
  from cudf import Series
  from cudf.core.dtypes import Decimal32Dtype, Decimal64Dtype, Decimal128Dtype
  from cudf.testing import _utils
  from cudf.testing._utils import NUMERIC_TYPES, assert_eq, expect_warning_if, gen_rand
label_desc: Use image node_id 2 for calling a global function with example usage: test_sum_string() without return types